What size is you carboy? If it is the carboy that came in a kit with a fermentation bucket and a bottling bucket, then it is probably 5 gallon and was meant for using as a secondary. That's how my kit was. You can use a 5 gallon carboy as a primary, just keep in mind that you may want to either brew a smaller batch or plan for some loss to blowoff. Definitely rig a blow off tube on it for the first week or so until fermentation slows down.